Certain properties of gypsum (calcium sulfate dihydrate) make it very popular for use in making industrial and building plasters and other building products; especially gypsum wallboard. It is a plentiful and generally inexpensive raw material which, through a process of dehydration and rehydration, can be cast, molded or otherwise formed to useful shapes. It is also noncombustible and relatively dimensionally stable when exposed to moisture. However, because it is a brittle, crystalline material which has relatively low tensile and flexural strength, its uses are typically limited to non-structural, non-load bearing and non-impact absorbing applications.
Gypsum wallboard; i.e. also known as plasterboard or drywall, consists of a rehydrated gypsum core sandwiched between multi-ply paper cover sheets, and is used largely for interior wall and ceiling applications. The paper cover sheets contribute significantly to the strength of plasterboard, but, in doing so, compromise its fire resistance. Furthermore, because of the brittleness and low nail and screw holding properties of its gypsum core, conventional drywall by itself cannot support heavy appended loads or absorb significant impact.
Accordingly, means to improve the tensile, flexural, nail and screw holding strength and impact resistance of gypsum plasters and building products have long been, and still are, earnestly sought.
Another readily available and affordable material, which is also widely used in building products, is ligno-cellulosic material, particularly in the form of wood and paper fibers. For example, in addition to lumber, particleboard, fiberboard, waferboard, plywood and hardboard (high density fiberboard) are some of the forms of processed ligno-cellulosic material products used in the building industry. Such materials have better tensile and flexural strength than gypsum. However, they are also generally higher in cost, have poor fire resistance and are frequently susceptible to swelling or warping when exposed to moisture. Therefore, affordable means to improve upon these use limiting properties of building products made from cellulosic material are also desired.
Previous attempts to combine the favorable properties of gypsum and cellulosic fibers, particularly wood fibers, have had very limited success. Attempts to add cellulosic fibers, (or other fibers for that matter), to gypsum plaster and/or plasterboard core have generally produced little or no strength enhancement because of the heretofore inability to achieve any significant bond between the fibers and the gypsum. U.S. Pat. Nos. 4,328,178; 4,239,716; 4,392,896; and 4,645,548 disclose recent examples where wood fibers, or other natural fibers, were mixed into a stucco (calcium sulfate hemihydrate) slurry to serve as reinforcers for a rehydrated gypsum board or the like. Similarily, attempts to add gypsum particles to wood fiber products have been disappointing due to the inability to retain enough gypsum in the product to materially improve the fire-resistance or dimensional stability of the base material.
Recently, several manufacturers have had limited success in producing board products comprising a combination of gypsum and wood, or paper fibers. In several of these processes, calcined gypsum (stucco) is mixed with wood or paper fibers and water to make a slurry, then pressed while, or before, the stucco rehydrates to solidified gypsum.
In one such process (Prior Art Process A) waste paper is mixed with stucco in an aqueous slurry which is discharged onto a felting conveyor and dewatered. The thin hemihydrate/paper cake is wound convolutely onto a cylinder, to build up a board thickness, then cut to length. The green felts are stacked on carts between sheets of hardboard and allowed to hydrate over about a 3 to 4 hour period. The set boards are then dried, trimmed, and sanded and sealed as necessary.
In a so-called "semi-dry" process (Prior Art Process B) stucco and waste paper are mixed together dry. Part of the water needed for rehydration is added in a second mixer, and the mixed material formed into various layers on a continuous running belt. The remainder of the required water is sprayed onto the several layers which are then combined into a multi-layer mat prior to entering a continuous press. After the initial "set", the raw boards are cut and trimmed, allowed to "fully set" on a holding belt, and then dried.
In another so-called "semi-dry" process (Prior Art Process C) stucco and wood flakes are pre-mixed dry. Water, in the form of ice or snow crystals is metered into the mix, which is then spread onto an endless mat on the bottom of a continuous press. The ice melts slowly after compression of the mat to the desired thickness and then hydration takes place. After the board finally sets, it is cut, trimmed and dried. Sanding is probably also desirable, if not necessary.
Examination of commercial boards from these processes reveals that they consist of a compacted mixture of discrete gypsum and fiber materials, i.e. they are more a physical mix than a homogeneous composite. While it might be said that the gypsum provides, or serves as, a binder for the fibers in these boards, it does not appear that there is any appreciable direct physical interlocking or chemical bonding between the gypsum crystals and the fibers. Furthermore, whether because of the way in which these boards are formed, or because of the mechanical mixing of gypsum crystals and fibers, and/or because of the clumping of the paper fibers or stucco, these boards do not exhibit good homogeniety and uniformity of properties; i.e. such as density and strength, over their expanse.
According to a process (Prior Art Process D) described in recently issued U.S. Pat. No. 4,734,163, raw or uncalcined gypsum is finely ground and wet mixed with 5-10% paper pulp. The mash is partially dewatered, formed into a cake and further dewatered by pressure rolls until the water/solids ratio is less than 0.4. The cake is cut into green boards, which, after being trimmed and cut, are stacked between double steel plates and put into an autoclave. The temperature in the autoclave is raised to about 140.degree. C. to convert the gypsum to calcium sulfate alpha hemihydrate. During the subsequent incremental cooling of the vessel boards, the hemihydrate rehydrates back to dihydrate (gypsum) and gives the board integrity. The boards are then dried and finished as necessary.
This process is distinguisable from the earlier ones in that the calcination of the gypsum takes place in the presence of the paper fibers.